Alkaline electrolysis is an electrochemical process which dissociates water into hydrogen and oxygen by means of applying electric current. It is carried out in an electrolyzer comprising a tank, a cathode, an anode and two compartments separated by a porous and semipermeable diaphragm, repeatedly stacked. Water splitting is due to two redox half-reactions occurring at the electrodes, hydrogen being generated at the cathode and oxygen at the anode. The overall reaction can be described in the following manner:2H2O(l)→2H2(g)+O2(g); E0=+1.229 V
The necessary ion exchange is carried out by means of using an electrolyte.
Therefore the reduction reaction occurs at the cathode (with a negative charge) where the water is dissociated into hydrogen gas and hydroxyl ions. Said ions migrate through a diaphragm towards the anode (with a positive charge) where they are oxidized to yield water, oxygen and two free electrons which migrate towards the cathode, closing the circuit. The resulting gases are collected at the respective electrodes.
The electrolyte used in this type of electrolyzer is generally sodium or potassium hydroxide. The ionic conductivity of said electrolyte depends on its concentration, the concentration for which said conductivity is the maximum at the working temperature being chosen. The role of the electrolyte is to close the electric circuit allowing the passage of ions but not electrons between both electrodes.
As has been mentioned previously, the anode and cathode are separated by a porous and semipermeable diaphragm which must allow the passage of electrolyte (OH−) and prevent, however, the gas bubbles from permeating through it thus preventing said gas bubbles which are generated in each cell (H2 and O2) from recombining to reproduce water, which can results in explosive mixtures since it is a highly exothermic reaction. The diaphragm causes molecular sieving as both gases go from one chamber to the other, which gases, once having been generated at the corresponding electrode, rise to the top of the electrolyzer due to their lower density compared with that of the electrolyte. The energy necessary for carrying out said electrolysis is supplied in the form of electric energy.
The requirements which must be met by the diaphragms for effective and safe application in alkaline electrolysis are very restrictive. They must show sufficient mechanical strength and chemical resistance, considering the highly alkaline conditions of the electrolyte used (i.e., NaOH, KOH (20-33 wt. %) at 40-90° C.). They must further be porous and semipermeable, such that they allow the flow of the electrolyte but not that of the generated gases (H2/O2), i.e., they must have a high bubble point pressure for said gases. The diaphragms must be hydrophilic and have low electrical resistance and high ionic conductivity. Logically, the materials making them up must preferably be environmentally friendly. Since the efficiency of the electrolysis increases by increasing the temperature, it is also desirable that the diaphragms withstand the corrosive conditions of the electrolyte over long time periods and preferably at high temperatures (up to 150° C.).
In the state of the art various materials are used for manufacturing different diaphragms, but in general, none satisfactorily meet all the mentioned requirements, some even having serious drawbacks. Some of these materials and their drawbacks are shown below.
Asbestos fibers, for example, have been used in membrane manufacturing but they were quickly replaced with other materials after it was demonstrated that they are very harmful to the environment.
Polytetrafluoroethylene (PTFE) and its derivatives have been used for manufacturing diaphragms but since they are highly hydrophobic polymers, it is necessary to add considerable amounts of wetting agents to them to increase their hydrophilicity, such as zirconium oxide, titanates, and doped titanates, which complicates their production method. Furthermore, the use of these diaphragms has the drawback that the efficiency of the electrolysis process is very low and the energy necessary to produce hydrogen is high.
The same occurs with other polymers, such as polyfluoroethylene propylene, polyarylethersulfone, polyperfluoroalkylvinyl ether, polyphenyl sulfur, which need to incorporate ceramic materials, such as zirconates or titanates to improve their wetting. Their manufacturing method is very complex and requires techniques such as cold rolling or by means of casting. In the case of the mentioned polymers, their functionality is limited.
It has been demonstrated that microporous polysulfone diaphragms can be used in alkaline electrolyzers; nevertheless, they have the drawback that their hydrophilicity is reduced.
Commercial membranes such as Zirfon® (polysulfone with zirconium oxide microparticles) have been designed for solving said hydrophobicity limitation, but have some drawbacks such as, for example, their production, which is very costly.
Nickel oxide (NiO) has also been used for manufacturing diaphragms but it is potentially toxic for the environment due to the Ni leaching potential.
Cermet composite materials (metal materials containing nickel and ceramic oxide particles embedded in a mesh of said metal) have been used for manufacturing diaphragms but have the same environmental drawbacks previously described for Ni leaching.
Metal materials have the limitation of being electrically conductive, which makes their incorporation on a electrolytic cell with zero-gap configuration unattainable. Furthermore, only the noble metals from the platinum group have sufficient chemical resistance for said application, however, neither Ni nor Zr are sufficiently passivated to assure the long-term stability in electrolytic conditions for diaphragms manufactured by sintering.
Self-supporting ceramic materials, i.e., those not needing a matrix of another material which provides mechanical strength to the set, such as zirconium oxide, titanates and zirconates (i.e., NiTiO3, ZrO2, TiO2, BaZrO3, BaTiO3, CaZrO3, CaTiO3) can be sintered in controlled conditions to achieve suitable microporosity allowing the flow of fluids (electrolyte) but not that of the gas bubbles generated in the electrolyzer. These materials have the required chemical resistance, but low mechanical strength since they are easily friable and peel-off. Sintering a diaphragm made up of only defect-free (macropores) large ceramics is technologically complex. Furthermore, the different coefficients of expansion between a ceramic material (the diaphragm) and a metal material (the electrolyzer casing) make preventing cracks in the ceramics after being subjected to high temperatures very difficult.